1. Field of the Invention
The present invention relates to an integrated thin film solar battery including a plurality of unit cells serially connected with one another on a same translucent insulating substrate and a method for fabricating the same. More particularly, the present invention relates to an integrated thin film solar battery where a transparent conductive film electrode of one unit cell is connected to a back electrode of an adjacent unit cell via a contact line, and a method for fabricating the same.
2. Description of the Related Art
As one of conventional thin film solar batteries, a solar battery cell including a transparent conductive film electrode made of SnO.sub.2, ZnO, or ITO, a photoelectric conversion layer, and a back reflection metal film electrode formed in this order on a translucent insulating substrate is known. Glass or a heat resistant resin such as polyimide is used as the translucent insulating substrate.
An integrated thin film solar battery is composed of such thin film solar battery cells serially connected with one another on a large-area substrate, to obtain a thin film solar battery cell unit with high voltage and high output.
Such an integrated thin film solar battery is fabricated in the following manner. First, a transparent conductive film is formed on a translucent insulating substrate. The transparent conductive film is then patterned by forming insulating patterning lines which divide the transparent conductive film into a plurality of strip-shaped transparent conductive film electrodes.
A photoelectric conversion layer is then formed on the transparent conductive film electrodes and patterned by forming patterning lines at positions displaced from the insulating patterning lines of the transparent conductive film so that only the photoelectric conversion layer can be partially removed to form grooves without damaging the underlying transparent conductive film electrodes. Subsequently, a back reflection metal film is formed on the resultant substrate, and patterned by forming patterning lines at positions opposing to the insulating patterning lines of the transparent conductive film with the patterning lines of the photoelectric conversion layer interposed therebetween so that only the back reflection metal film can be divided in an insulating manner, to form back reflection metal film electrodes. In this way, the integrated thin film solar battery is obtained.
With the technique described above, the transparent conductive film electrode of one thin film solar battery unit cell can be serially connected to the back reflection metal film electrode of an adjacent thin film solar battery unit cell. In this way, a number of unit cells are serially connected with one another, realizing the integrated thin film solar battery formed on a large-area substrate.
The insulating patterning of the transparent conductive film, as well as the patterning of the photoelectric conversion layer for forming grooves for contact lines, are conventionally performed by a scribing technique using a laser beam. This technique is considered optimal among currently available techniques for the following reasons: It is advantageous in the processing precision and the tact time (a time required to process one large-area substrate); and it allows for selective film processing utilizing the selective absorption of laser light based on a wavelength of the laser light by an object to be processed.
A conventional technique for forming the patterning lines for the photoelectric conversion layer is disclosed in Japanese Publication for Opposition No. 4-47466. According to this technique, the photoelectric conversion layer is irradiated with a laser beam having a wavelength of 0.60 .mu.m or less, to partially abrade the irradiated portions, forming grooves.
According to the above technique, the photoelectric conversion layer can be processed without damaging the underlying transparent conductive film electrodes because the laser beam passes through the transparent conductive film electrodes. Examples of the laser beam having a wavelength of 0.60 .mu.m or less used conventionally include second harmonic (SHG, 0.532 .mu.m) of an Nd-YAG laser beam, an XeCl excimer laser beam (0.308 .mu.m), and an Ar laser beam (0.51 .mu.m). Recently, second harmonic (SHG, 0.532 .mu.m) of an Nd-YAG laser beam tends to be used since it is easy to maintain, does not use corrosive high-pressure gas, and is inexpensive to operate.
A problem arises in the mass-production of such a large-area integrated thin film solar battery. That is, the tact time increases in the patterning of the photoelectric conversion layer.
More specifically, in the patterning of the large-area integrated thin film solar battery, the patterning pitch, i.e., the width of each thin film solar battery unit cell is generally in the range of 5 to 15 mm, although it depends on the sheet resistance of the transparent conductive film electrodes, the contact resistance between the photoelectric conversion layer and the back reflection metal film electrodes, and the shape of the photoelectric conversion layer.
Accordingly, when the length of the translucent insulating substrate in the direction perpendicular to the patterning lines is 450 mm, for example, and the patterning pitch is 10 mm, 45 patterning lines are formed. The number of patterning lines is greater as the size of the substrate becomes greater, increasing the tact time required to process one large-area substrate.
The above problem can be overcome by splitting the laser beam into a plurality of laser beams by use of a beam splitter, to allow several laser beams to be used simultaneously and thus suppress the increase in the tact time.
However, the energy amount required to abrade the photoelectric conversion layer does not change even though a split laser beam is used. Therefore, the original laser beam before the splitting is required to have the energy equal to the energy required for one split laser beam multiplied by the number of split laser beams.
The second harmonic (SHG, 0.532 .mu.m) of the Nd-YAG laser beam is generated by transmitting a fundamental wave (1.064 .mu.m) of the Nd-YAG laser beam through nonlinear optical crystal (e.g., BBO crystal). At this time, the energy of the resultant SHG beam is reduced to about one-tenth of that of the original fundamental wave. Accordingly, the number of split laser beams obtained from this laser beam is limited.
In consideration of the above, a structure of a large-area integrated thin film solar battery, as well as a method for fabricating such a solar battery, capable of shortening the tact time and thus realizing improved production efficiency and cost reduction are earnestly required.